The battery system of high voltage (at least 60V) having the plurality of cells connected in series/parallel has a voltage deviation between the cells occurred due to cell production process, capacity difference and self-discharge effect before/after being driven, as well as a voltage deviation between the cells occurred due to temperature irregularity within the battery system and structure difference in the battery packs upon being driven.
Such voltage deviation results in hindering battery voltage irregularity in accordance with operations of the battery mounted in a vehicle, further leading to battery deterioration and lifetime reduction.
Therefore, such high-voltage battery system reduces the deviation between the cells by mounting a cell balancing circuit. The cell balancing circuit is connected to a Battery Management System (BMS) which is a battery controller. The cell balancing circuit includes a Passive Cell Balancing circuit of a resistor discharge method and an Active Cell Balancing circuit of a DC/DC converter.
The existing Passive Cell Balancing circuit obtains information on the voltage deviation between the cells through a real-time voltage measurement, and determines whether the balancing operation is performed or not based on the information. In other words, it is a closed-loop circuit of voltage feedback method which causes start and end of balancing to be determined by real-time voltage sensing.
Korean Patent Application Publication No. 2006-0083343 relates to battery equalization method of battery pack provided with a plurality of batteries, which comprises a step of detecting a voltage for each of a plurality of batteries, a step of computing a mean value of the plurality of battery voltages detected, a step of setting a balancing target range based on the voltage mean value computed, a step of determining whether or not a battery to be balanced exists of which the battery voltage detected is contained in the balancing target range, a step of selecting the highest voltage battery and the lowest voltage battery of the batteries to be balanced if it is determined that the battery to be balanced exists, and a step of balancing the cell in such a way that the highest voltage battery is discharged and correspondingly the lowest voltage battery is charged until the voltage mean value is reached by connecting the highest voltage battery and the lowest voltage battery in parallel.
Korean Patent Registration No. 0680901 relates to a battery management system which is provided with a master module and a plurality of slave modules, which comprises a step of confirming a mode of a battery pack which each slave module manages in a master module, a step of selecting a lowest cell voltage by monitoring a cell voltage of each battery cell configured within the battery pack if the battery pack is in any one of charging mode, rest mode or discharging mode as a result of confirmation, a step of comparing the selected lowest cell voltage with the cell voltage of each battery cell monitored to perform discharge for a prescribed discharge time for the battery cell in which the voltage difference is greater than a predetermined threshold, and a step of cell balancing the battery pack by repeating the step of selecting the lowest cell voltage, the step of confirming the threshold range and the step of performing discharge until the cell voltage between each battery cell is not more than the threshold.
U.S. Laid Open Patent No. 2007-0194791 relates to a method for monitoring a battery state by measuring an internal resistance, which comprises a step of measuring each of a voltage applied to the battery and a voltage applied to a first external resistor after connecting the battery cell with the first external resistor, a step of measuring each of the voltage applied to the battery and a voltage applied to a second external resistor after connecting the battery cell with the second external resistor, a step of computing a resistance value of the battery using a current value computed by the voltage applied to the first external resistor and the voltage applied to the second external resistor, the voltage applied to the battery when the first external resistor is connected, the voltage applied to the battery when the second external resistor is connected to compare it with predetermined resistance value, and a step of issuing an alarm if the battery has abnormal resistance value.
U.S. Pat. No. 6,424,157 relates to a method of monitoring a state of a battery disposed in a vehicle, in which the method computes a battery dynamic internal resistance IR, a dynamic polarization resistance PR, Quiescent voltage and computes correct state of charge (SOC) based on the computed values, based on a voltage value using a sensor measuring a voltage of the battery when the engine is started up and a current value measured using a sensor measuring a current drain of the battery when the engine is started up.
WO 2005/055358 relates to a method of equalizing batteries connected in series, in which the method comprises a step of measuring charging level of the battery cells connected in series, a step of performing a balancing step by adjusting a charging current level of each cell in accordance with the charging level of each cell and a step of equalizing the batteries by repeating the steps.
However, if the balancing is performed by measuring the battery voltage as in Korean Patent Application Publication No. 2006-0083343, the battery voltage may contain properties of an electromotive force, a voltage of a resistor component, and a polarization voltage caused by reaction to generate nonlinear voltage according to an amount of currents upon being driven. In other words, the terminal voltage of the battery is represented by summing the electromotive force, the IR voltage and the polarization voltage. However, the current frequently flows in a state where the vehicle is driven and thus the voltage measured at a terminal is different from the battery electromotive force due to actions of the IR voltage and the polarization voltage. Even if the current does not flow through the vehicle, the balancing current flows due to the cell balancing itself, whereby correct electromotive force can not be achieved.
A main target of the cell balancing is to remain the electromotive force between the cells at the same level. If the balancing current flows, however, it is difficult to correctly control the electromotive force at the desired level by a cell balancing logic based on real-time terminal voltage.
The cell under the course of balancing can have a bad influence since a reverse phenomenon can be happened which makes lost electromotive force greater than the target cell voltage due to a terminal voltage overestimated than the electromotive force.